Real time seismic telemetry systems require a plurality of data acquisition units to be disposed along a seismic line for sensing seismic waves in the earth's crust. Normally, these seismic waves are artificially induced to create a wavefront that can be measured as it passes the remote data acquisition units. The data that is collected is normally in analog form which is obtained from a transducer such as a geophone or a hydrophone. Each of the remote data acquisition units is oriented such that they take data at a plurality of points disposed remote from a central shock creating device. By examining this data, scientists can utilize analytical models of the structure of the substrata in the locale of the transducers.
In order to collect all of the data from the data acquisition units, the central location must be able to recognize the particular data acquisition unit from which the data has been received and store this data in an appropriate medium in association with the location data. The format for the storing of this data has been provided by the Society of Exploration Geophysicists such that the data can be analyzed and correlated at a later time. This data is normally stored as digital words in a predetermined way. Therefore, it is necessary for the central station to arrange the data from the various data acquisition units according to the proper format.
To achieve proper time correlation of the data after reception and storage, it is necessary that all of the sampled data for a particular sample in time be stored such that retrieval from the digital form allows correlation of the data without introducing a time error between the samples. For systems which receive all of the analog data at a central system in a multiplex form, this presents very few problems with receiving the data and processing since systems of this type normally utilize a common clock, that is, all data acquisition units are synchronized to the same clock. Normally, systems of this type are hardwire systems which require a separate line for the clock and possibly a separate line for some control signals.
Hardwire systems sometimes are inappropriate for the application and a wireless system must be utilized. For these systems that utilize a real time data link, the analog data must be modulated onto a carrier and transmitted over a plurality of discrete channels and then multiplexed at the central station. Since only certain portions of the frequency spectrum are available for use, the portion chosen must be relatively free for use by the transmitters, that is, a relatively dedicated channel. Unfortunately, there are very few portions of the frequency spectrum that can be allocated to a particular use, thus requiring a system to operate in a relatively narrow portion of the spectrum.
In order to operate a large number of discrete channels over a narrow portion of the frequency spectrum, the modulation technique utilized is of prime importance. Most of the efficient modulation techniques utilize transmission of digital words in either a frequency shift key (FSK) or phase shift modulation (PSM). The fundamental frequency for these modulation techniques is normally the bit rate clock of the digital word. This, of course, depends upon the number of bits transmitted for each digital word, each digital word being transmitted at the sampling rate thereof. Since the sampling rate is on the order of 1 KHz, the bit rate of the digital word is much higher than this rate and is determined by the number of bits that is required to encode the data with the proper resolution and the number of bits required for synchronization of the word at the central location. Since the clock that is used to generate the digital word at the data acquisition unit is not available to the central location at which the digital word is processed, it is necessary for the central location to recognize the beginning and end of each of the digital words. This is normally done by including a number of synchronizing bits that precede each digital word. However, the more synchronizing bits that are required to be present in the digital word, the higher the bit rate clock must be. This, in turn, increases the transmission bandwidth that is required for transmission of the digital data. Therefore, it is necessary to minimize the number of bits transmitted while retaining the data resolution therein to conserve bandwidth.
Upon reception of the data, it is necessary to receive the signal over the data link and achieve bit synchronization and word synchronization in as short a time as possible. This is due to the fact that in some instances the initial data transmitted is important and must be retrieved. In addition, it is necessary that the central location maintain synchronization with all of the remote data acquisition units at all times during transmission of the data in order to achieve a high degree of accuracy for each measurement taken. Such things as noise and the distance of the data acquisition units from the central location can cause noise to interfere with reception of the data. This noise results in "garbled" data that is useless for correlation or other purposes and must be taken into consideration.
Once the data is received, it is necessary, as described above, to store the data in digital form. If the data is transmitted in digital form, it is relatively easy to store the data in this format in the storage medium. However, a problem exists in that all of the clocks that generate the bit frequency in each of the remote units are not synchronous, that is, they are asynchronous. Although the digital words are received on a common time base, storage of the digital words onto the storage medium erases any time base to determine the "skew" between the different remote units. This lack of skew information results in a system error when the data is reconstituted and analyzed since any central processing system will assume that all the clocks of the remote units are synchronous. It is therefore necessary to provide some type of correction factor or signal to indicate what the skew actually is such that, upon reconstitution of the signal, the skew between various remote units can be determined.
In view of the above, there exists a need for a real time seismic telemetry system that provides a data acquisition unit for disposal at a remote location that operates over a wireless data link and occupies a very narrow portion of the allowed bandwidth for the entire system. The central processing system for receiving the data from the remote units must have the capability to retrieve the data in as complete a form as possible and store this data on a storage medium taking into effect the relative skew between the various remote units. In addition, the modulating system used over the data link must provide very efficient use of the allocated bandwidth.